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Suzuki motorcycle GS850G: Motorcycle: 1980 GS850G.I'm experienci…

Customer Question

Motorcycle: 1980 GS850G. I'm experiencing...

Motorcycle: 1980 GS850G.I'm experiencing a significant loss of power that seems to be getting worse, and I have fuel pooling in the air box when the engine runs. Carbs have been rebuilt and the float height settings double checked. Also replaced all four spark plug resistor caps, which didn't help.I would like to find a motorcycle specialist who is sharp on the vacuum-operated carb and airbox setup.Thanks.-Nate

Carbs only leak fuel when there is a problemwith the float, needle valve under the floator plugged float bowl vents or overflow tube.What likely occurred is that you have some badstuff coming down the fuel lines from the tankor a faulty petcock or bad fuel hose.Without a good fuel filter inline the float valveswill immediately start to stick and leak fuel.The float valves have to be in perfect conditionand the seat has to be very clean as well.Some needle valves have orings on themwhich can also leak fuel into the bowl makingit overfull.If any fuel runs from the carbs when the bikeis not running then the float needles are leaking.If you have a vacuum petcock it could be faultyand may be allowing raw fuel to be pulled into the enginethrough the vacuum hose.Even if they were just cleaned the carbs have to havean external fuel filter or they will cause problemsimmediately and have to be cleaned again, at leastthe float valves.-----

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Customer reply replied 2 years ago

Thank you for your fast response.The carbs on the bike came from a 1981 GS850GL. Float height settings were double checked. I also did a hot water test on all four floats, but it wasn’t obvious if anything was wrong. Two of the floats did emit more bubbles than the other two, so I replaced them using floats salvaged from the original carbs. I didn’t notice any difference after that.All four needle valves were replaced with new ones in the carb rebuild. I checked all four of them while I was doing the float checks, and all appeared to still be in perfect condition. All four o-rings for the needle valve housings were also new, and they were still in place and intact. No debris of any kind was found that could have kept the needle valves from seating. No debris was found in the bowls; they were still very clean. Also, I’ve never had a problem with fuel leakage when the bike is not running.The tank was cleaned and sealed using a Caswell epoxy kit.The petcock was rebuilt. It does have a pretty fine mesh screen which should serve as an inline filter.So I still have these items to check:• Overflow tube plugged?• Float bowl vents plugged?• Shine flashlight in the tank. Any debris?• Check petcock screen.• Petcock vacuum valve.What is the best way to check the petcock vacuum mechanism, to make sure it’s not pulling in fuel?So I know I’m looking in the right place, where are the float bowl vents and overflow tube?Also, do you know if the jetting would have been different on the 1981 GS850GL that my carbs came from? Last Sunday I ran the bike at idle with the air box removed. When I placed my hand over any of the four carb inlets, fuel burbled out the inlet into my hand. That doesn’t seem normal to me. What are your thoughts on that?-Nate

Used a #115 main jet on both years.If the petcock vacuum diaphragm is leakingyou will have fuel in the vacuum line.Also the petcock may not work very well.The float levels could be a little highdepending on how they were set.They should be measured without depressing thespring loaded end on the float needles if it has those.Fuel height in the bowl is much more accuratethan float level. 5mm below top of the bowl isstandard.Covering the carbs air intake is like chokingthe air supply so the engines vacuum draws in morefuel from the main jet area. This is why fuelcomes up and out the carb inlet when you do that.If any carbs drip fuel when stopped then thefloat needles or orings are not sealing orfloats are not floating or you have some otherproblem. There is often a brass tubein the bottom of the float bowls. This is the overflowwhen the carb is too full of fuel.If these develop cracks down lower the carbs can leak.Big no-no is depending on the petcock screento protect your carbs from debris.They need a fine paper or gauze type inline filteravailable at auto or cycle stores.This will stop alot of problems with leaky carbs.If your air box fills with fuel the fuel is likelycoming from the carbs being overfull if theneedles are all sealing solid.I have seen aftermarket needles leak and alsofloats hitting on the bowl gaskets when madepoorly.Replace the float needle valve seat orings as that isa major source of problems. They might lookgood but any shrinkage will ruin them.Fuel height in the bowl can sometimes be checked from outside the carbsusing a hose on the float bowl drain.Get this manual:http://www.mtsac.edu/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdfLet me know if the carbs leak when engine is off or just needadjustments. Check that all cylinders (pipes) are running the same heat.Air box is necessary for it to run well and must be sealedas new were.------

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Customer reply replied 2 years ago

Here are responses to the items you discussed, and some additional information:• The petcock does seem to be working fine. The bike actually ran very well just a few weeks ago.• The float heights were set correctly, without pressing down the needle springs.• No fuel dripping with the engine not running.• #2 and #3 are not hot like #1 and #4 when idling. The 2-3 ignition coil checks OK at 11,976 ohms, and all 4 5000-ohm resistor caps have been replaced with NGK's.• The 4 inlet boots going into the air box are kind of stiff. I think they’ve gotten worse from being in contact with fuel. One or two of these fit loosely in the air box.• I believe the sealing inside my air box has been compromised by contact with fuel.Thanks for the link to the manual. I will take a look at it and check the remaining items.

Is it possible to swap the coils to see if thecylinders 2 and 3 run better then and check forair leaks around the rubber carb inlets?Cylinder 2 and 3 sound like they are not pullingtheir weight so to speak.Also the carb butterflies have to be synchromizedto open at the same time as outlined in the carbmanual I believe. Carbs 1 and 4 may be openingsooner than 2 and 3.Hard or leaking rubber carb boots can cause lean fuelmixtures which may not burn effectively.Good carb boots between carbs and engine are a must.The air box boots may have less effect but still some.The engine compression should be close to even as possibleand valve clearance could be an issue in weak cylinders.---------------

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Customer reply replied 2 years ago

Unfortunately, the connectors for the primary coils for 2-3 are different from 1-4, so it would be possible, I think to swap them, but it will take some trickery.These carbs have been bench synchronized only so far, using a piece of wire to make the openings even when on the idle stop. I'm saving the full running synchronization until I get this fuel and air box issue sorted out.Earlier today, I started going down my list of things to check, and with the air box removed, I discovered that I had fuel running out of #3 and #4 carbs as soon as I switched the petcock to PRI. I was just trying to perform a fuel level check on #1 when this happened. I did this check before with the airbox still on, and I didn't see fuel leaking from any of the carbs. I can't explain that yet. Anyway, I ended up pulling the gang of carbs and separating #4. I removed the bowl and still didn't see anything abnormal. I then performed a fuel level check on #4 with it in a soft-jawed vice. I think the check came out normal, right at .203" (13/64"). My manual says they should be .18" to .22", so mine is pretty much dead nuts. I'm doing this with the bowl and gasket seated, and I'm measuring to the bottom of the main housing, where the main housing meets the bowl. The main housing overlaps the top of the bowl by about 3/64", so if I measure to the actual top surface of the bowl, I get .25". Which method is correct? My Suzuki factory manual also says to do this check on the bike with it idling, but the carb rebuild manual you linked me to shows a single carb in a vice, like the way I'm doing it. ShoulWith the way the fuel poured out of #3 and #4 with the carbs still on the bike, I expected to see something like that

Customer reply replied 2 years ago

My posting got messed up so I re-did it here:Unfortunately, the connectors for the primary coils for 2-3 are different from 1-4, so it would be possible, I think to swap them, but it will take some trickery.These carbs have been bench synchronized only so far, using a piece of wire to make the openings even when on the idle stop. I'm saving the full running synchronization until I get this fuel and air box issue sorted out.Earlier today, I started going down my list of things to check, and with the air box removed, I discovered that I had fuel running out of #3 and #4 carbs as soon as I switched the petcock to PRI. I was just trying to perform a fuel level check on #1 when this happened. I did this check before with the airbox still on, and I didn't see fuel leaking from any of the carbs. I can't explain that yet. Anyway, I ended up pulling the gang of carbs and separating #4. I removed the bowl and still didn't see anything abnormal. I then performed a fuel level check on #4 with it in a soft-jawed vice. I think the check came out normal, right at .203" (13/64"). My manual says they should be .18" to .22", so mine is pretty much dead nuts. I'm doing this with the bowl and gasket seated, and I'm measuring to the bottom of the main housing, where the main housing meets the bowl. The main housing overlaps the top of the bowl by about 3/64", so if I measure to the actual top surface of the bowl, I get .25". Which method is correct? My Suzuki factory manual also says to do this check on the bike with it idling, but the carb rebuild manual you linked me to shows a single carb in a vice, like the way I'm doing it. Should running or not running make a difference in the level check?With the way the fuel poured out of #3 and #4 with the carbs still on the bike, I expected to see something like that in my off-bike level check. But it just came out normal. My theory on that is that when it’s off the bike, I don’t have the higher fuel pressure exerted on the needle valve that I do when it’s being fed by the nearly full tank on the bike. So it must be able to unseat the needle when on the bike, but not when in the vice. What are your thoughts on that? Should I try adjusting the float tab to the extreme low-level (lean) end of the spectrum? Or maybe even a bit lower than that?

If the needles leak then that is the problem.Even with a full tank of gas or even a fuelpump the needles should never leak.If they leak even occasionally that is notacceptable. You might have some bad aftermarket needles.The originals are much better but are hard to find.The fuel height should be just a little undera full bowl and carbs will not flood unless thefloat needle is not sealing properly.You could try seating the needles with some finegrinding compound. Just like resurfacing an engine valve.Spin the needle until it seats evenly in the valve seat.The leaks will prevent you from ever getting it tuned right.----

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